1. Field of the Invention
The present invention relates to a semiconductor device with a power supply circuit mounted thereon, and a liquid crystal device and electronic equipment using the semiconductor device. Particularly, the present invention relates to a technique for the prevention of malfunctions which may occur in the case of power supply emergencies, such as in the case where a battery is taken out.
2. Description of the Related Art
In a liquid crystal device such as a liquid crystal display, a display operation is implemented when a voltage is applied to liquid crystals which are sealed between substrates in which electrodes are formed. This type of liquid crystal display has been widely used in recent years in various electronic equipment such as personal computers, word processors, cellular phones, electronic pocketbooks, and the like.
Such electronic equipment using a liquid crystal display is designed so that the screen becomes blank instantaneously when the power source is cut off according to a prescribed sequence. The phenomenon of instantaneous lighting occurs when the display is turned off by a procedure other than the above-described sequence, such as the case in which the battery is abruptly drawn out during the display operation, or the electronic equipment is forcedly terminated. Specifically, in this phenomenon the screen image instantaneously disappears when the battery is drawn out during display, following which lighting images such as horizontal lines appear for a while.
The present inventors have conducted extensive studies concerning the instantaneous lighting phenomenon and have achieved the present invention.
Accordingly, an objective of the present invention is to provide a semiconductor device with a power supply circuit mounted thereon, which can prevent malfunctions such as instantaneous lighting which occurs at the time of power supply emergencies, and a liquid crystal device and electronic equipment using the semiconductor device.
In one aspect of the present invention, a semiconductor device including a drive circuit, a drive control circuit which controls the drive circuit, and a power supply circuit which supplies a potential to the drive circuit and drive control circuit,
wherein the power supply circuit comprises:
a boosting circuit, to which a first power supply potential which is a ground potential from an external power supply and a second power supply potential which is a potential other than the ground potential are supplied, and raising the absolute value of the second power supply potential and charging the boosted potential to the capacitor; and
a bias generating circuit generating a potential supplied to the drive circuit and the drive control circuit based on the output potential of the boosting circuit, and
wherein the first power supply potential and a potential from the bias generating circuit are supplied to the drive circuit that outputs a potential selected from potentials supplied in accordance with a control of the drive control circuit during a normal power supply period, and, in the event of a power supply emergency in which an absolute value between the first and the second power supply potentials is lower than a given value, changes all potentials outputted from the drive circuit into the first power supply potential based on the signal activated in the event of the power supply emergency.
When a battery is drawn out and a power supply is forcibly cut off, for example, the first potential and the second potential which are supplied from external power supply sources become equal, e.g. the same potential as the ground potential, after a certain period of time.
Malfunctions such as instantaneous lighting occur because the time period required for the charge to be stored up to the capacitor of the boosting circuit after the forced cutoff of the power supply is longer than the period of time for the first and the second potentials to become equivalent.
In this instance, a potential equivalent to that discharged after cutoff of the power supply is supplied to the drive circuit and drive control circuit which receive a supply of potential from the power supply circuit including this boosting circuit. Malfunctions are caused by such a potential.
Therefore, in the event of a power supply emergency in which the absolute value between the first and the second power supply potentials is lower than a specified value, the drive circuit changes all potentials outputted from the drive circuit to a value equivalent to the first power supply potential (the ground potential) based on the signals which becomes active when a power supply emergency occurs. This causes all devices which are operated by a potential supplied by the semiconductor device to be completely shutoff without malfunction.
In another aspect of the present invention, a semiconductor device including a drive circuit, a drive control circuit which controls the drive circuit, and a power supply circuit which supplies a potential to the drive circuit and drive control circuit,
wherein the power supply circuit comprises:
a boosting circuit, to which a first power supply potential which is a ground potential from an external power supply and a second power supply potential which is a potential other than the ground potential are supplied, and raising the absolute value of the second power supply potential and charging the boosted potential to the capacitor; and
a bias generating circuit generating a potential supplied to the drive circuit and the drive control circuit based on the output potential of the boosting circuit,
wherein the first power supply potential and a potential from the bias generating circuit are supplied to the drive circuit that outputs a potential selected from potentials supplied in accordance with a control of the drive control circuit, and
wherein in the event of a power supply emergency in which an absolute value between the first and the second power supply potentials is lower than a specified value, the drive control circuit outputs a potential selecting signal that changes all potentials outputted from the drive circuit into the first power supply potential based on the signal activated when the power supply emergency occurs.
In this aspect of the present invention, the operation of the drive circuit in the first-mentioned aspect in the event of a power supply emergency is performed based on the potential selection signals from the drive control circuit.
The drive control circuit preferably comprises:
a logic circuit to which the first and the second power supply potentials are supplied, and outputting various logic levels;
a level shifter group to which a potential from the power supply circuit and the first power supply potential are supplied, including a plurality of level shifters for shifting the logic levels from the logic circuit; and
a potential selection circuit for outputting potential selection signals supplied to the drive circuit based on the output from the level shifter group.
This constitution of the drive control circuit ensures that both the first and the second logic levels from the logic circuit after a battery is drawn out become the same ground potential as each other. In this instance, although the output from the level shifter group may become indefinite, this can be controlled as described above, whereby occurrence of malfunctions can be prevented.
Preferably, the level shifter group has an input level setting circuit for setting the input to the level shifters at a specified value based on the signals which become active in the event of a power supply emergency regardless of the output of the logic circuit.
In this manner, indefinite output of the level shifter group can be prevented by setting the input to the level shifters at a specified value in the event of a power supply emergency. Malfunctions can then be prevented by controlling the device in the manner described above based on the specified value inputted to the level shifter group.
Preferably, the potential selection circuit has an output level setting circuit for setting the output of the potential selection circuit at a specified value based on the signals which become active in the event of a power supply emergency regardless of the output of the level shifter group.
In this manner, indefinite output of the level shifter group can be prevented by setting the output of the level shifters to a specified value in the event of power supply emergencies. Malfunctions can then be prevented by controlling the device in the manner described above based on the specified value outputted from the level shifters.
In this aspect and the previous aspect of the present invention, the signals activated in the event of a power supply emergency may be outputs from a comparator provided in the semiconductor device or power-on reset signals supplied from outside the semiconductor device.
In still another aspect of the present invention, a semiconductor device including a drive circuit, a drive control circuit which controls the drive circuit, and a power supply circuit which supplies a potential to the drive circuit and drive control circuit,
wherein the power supply circuit comprises:
a boosting circuit, to which a first power supply potential which is a ground potential from an external power supply and a second power supply potential which is a potential other than the ground potential are supplied, and raising the absolute value of the second power supply potential and charging the boosted potential to the capacitor; and
a bias generating circuit generating a potential supplied to the drive circuit and the drive control circuit based on the output potential of the boosting circuit, and
wherein the first power supply potential and a potential from the bias generating circuit are supplied to the drive circuit that outputs a potential selected from potentials supplied in accordance with a control of the drive control circuit during a normal power supply period, and
wherein the drive control circuit comprises:
a logic circuit to which the first and the second power supply potentials are supplied, and outputting a first logic level and a second logic level;
a level shifter group to which a potential from the power supply circuit and the first power supply potential are supplied, and shifting an output level from the logic circuit; and
a potential selection circuit for outputting potential selection signals supplied to the drive circuit based on the output from the level shifter group,
wherein:
each of level shifters forming the level shifter group comprises first and second circuits which are connected in parallel between a supply line for the first power supply potential and a supply line for a potential supplied by the power supply circuit;
the first circuit includes a first MOS transistor of primary conductive-type, a first MOS transistor of secondary conductive-type, and a second MOS transistor of secondary conductive-type which are connected in series to the first circuit, the first logic level from the logic circuit is supplied to gates of the first MOS transistor of primary conductive-type and the first MOS transistor of secondary conductive-type, and a potential between the first MOS transistor of primary conductive-type and the first MOS transistor of secondary conductive-type is a first output potential of each of the level shifters;
the second circuit includes a second MOS transistor of primary conductive-type, a third MOS transistor of secondary conductive-type, and a forth MOS transistor of secondary conductive-type which are connected in series to the second circuit, the second logic level from the logic circuit is supplied to gates of the second MOS transistor of primary conductive-type and the third MOS transistor of secondary conductive-type, and a potential between the second MOS transistor of primary conductive-type and the third MOS transistor of secondary conductive-type is a second output potential of each of the level shifters;
the second output potential is supplied to a gate of the second MOS transistor of secondary conductive-type in the first circuit, and the first output potential is supplied to a gate of the forth MOS transistor of secondary conductive-type; and
each of the level shifters further includes a potential maintaining circuit for maintaining the first and the second output potentials of each of the level shifters at a state before occurrence of a power supply emergency, in the event of a power supply emergency in which an absolute value between the first and the second power supply potentials is lower than a prescribed value.
Another cause of malfunction such as instantaneous lighting is a situation where the output from level shifters composing the shift level group becomes indefinite according to the input in the event of a power supply emergency.
According to this aspect of the present invention, in the event of a power supply emergency, the potential maintaining circuit which is provided in each level shifter maintains the first and the second output potentials of the level shifter which existed prior to the occurrence of a power supply emergency. As a result, the output of the level shifter group does not become indefinite at the time of a power supply emergency, whereby malfunctions due to a power supply emergency can be prevented.
The potential maintaining circuit provided in each of the level shifters forming the level shifter group may comprise:
a third MOS transistor of primary conductive-type connected in parallel with the first MOS transistor of primary conductive-type; and
a forth MOS transistor of primary conductive-type connected in parallel with the second MOS transistor of primary conductive-type.
In this instance, the second output potential may be supplied to a gate of the third MOS transistor of primary conductive-type, and the first output potential may be supplied to a gate of the fourth MOS transistor of primary conductive-type.
This constitution ensures that both the first and the second logic levels from the logic circuit become the same ground potentials as each other, after a battery is drawn out, for instance. In this instance, both the first and the second MOS transistors of primary conductive-type of the level shifter are either xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d.
The potential maintaining circuit turns either one of the third and fourth MOS transistors of primary conductive-type xe2x80x9conxe2x80x9d and the other xe2x80x9coffxe2x80x9d at the time of a power supply emergency. Specifically, when the state of first MOS transistor of primary conductive-type after a power supply emergency varies from the state before the power supply emergency, the first output potential causes the state of the third MOS transistor of primary conductive-type connected in parallel to be identical to the state of the first MOS transistor of primary conductive-type before the power supply emergency. In the same manner, when the state of the second MOS transistor of primary conductive-type after a power supply emergency varies from the state that existed before the power supply emergency, the second output potential causes the state of the fourth MOS transistor of primary conductive-type connected in parallel to become identical to the state of second MOS transistor of primary conductive-type before the power supply emergency. The first and the second output potentials from the level shifter before and after a power supply emergency can be maintained equivalent by this action. The previous output conditions can thus be maintained even when the first and the second logic levels from the logic circuit take the same logic conditions. Therefore, the first and the second output potentials from the level shifter can be fixed after a cutoff of the power supply. Thus, the drive circuit may cause all output potentials to become identical, for example, to the first power supply potential, whereby occurrence of malfunctions can be prevented.
It is preferable that the potential maintaining circuit provided in at least one of the level shifters forming the level shifter group includes a third MOS transistor of primary conductive-type connected in parallel to the first MOS transistor of primary conductive-type, and, when an on/off state of the first MOS transistor of primary conductive-type changes before and after the power supply emergency, the on/off state of the third MOS transistor of primary conductive-type is set identical to an on/off state of the first MOS transistor of primary conductive-type which existed before the power supply emergency.
The on/off state of the first MOS transistor of primary conductive-type varies before and after the power supply emergency in at least one level shifter. In this instance, malfunctions can be prevented by setting the state of the third MOS transistor of primary conductive-type connected in parallel to the first MOS transistor of primary conductive-type to a state identical to that of the first MOS transistor of primary conductive-type before the power supply emergency.
It is preferable that the potential maintaining circuit provided in at least one of the level shifters forming the level shifter group includes a fourth MOS transistor of primary conductive-type connected in parallel to the second MOS transistor of primary conductive-type, and, when an on/off state of the second MOS transistor of primary conductive-type changes before and after the power supply emergency, the on/off state of the fourth MOS transistor of primary conductive-type after the power supply emergency is set identical to an on/off state of the second MOS transistor of primary conductive-type before the power supply emergency.
The on/off state of the second MOS transistor of primary conductive-type varies before the power supply emergency in at least one level shifter. In this instance, malfunctions can be prevented by setting the state of the third MOS transistor of primary conductive-type connected in parallel to the second MOS transistor of primary conductive-type to a state identical to that of the second MOS transistor of primary conductive-type before the power supply emergency.
The present invention can be applied to a liquid crystal device and electronic equipment using the above-described semiconductor device, and can reliably prevent malfunctions which may occur in the case of a power supply emergency, such as the case where a battery is taken out.